Laser measuring devices are used for measuring target points which determine distances and directions (angles) of the target points. Laser measuring devices are known which are designed as a theodolite, a total station, a dumpy level, or a laser scanner. The laser measuring devices include a distance measuring device for measuring the distance between the target point and the distance measuring device and an angle measuring device for measuring angles. The measurement of the target points is supported by target objects which are placed at the target points. The target objects are used in surveying systems including laser measuring devices which transmit a search beam and a measuring beam, the wavelengths for the search beam and the measuring beam being advantageously different. The measuring beam is used for the measuring task of the laser measuring device, e.g., for measuring the distance and/or the angle, and the search beam is used to find and identify the target objects. Due to the different tasks of the search beam and the measuring beam, the search beam is strongly expanded for the purpose of detecting a larger spatial area, whereas a focused measuring beam is used for the measuring task. The target objects are subdivided into passive target objects which reflect the search beam of the laser measuring device and active target objects which do not reflect the search beam, but transmit a transmission beam. Active target objects have the advantage that the laser measuring device is able to distinguish the transmission beam, due to a different wavelength, from the beams which are reflected from reflective surfaces.
EP 1 734 336 A1 provides an active target object for a laser measuring device which transmits an infrared search beam and a measuring beam. The target object includes a reflector including at least one reflector element for reflecting the measuring beam, a receiver including a receiver element for receiving the infrared search beam, a transmitter including a transmitter element for transmitting a visible or an infrared search beam as well as a control device including an evaluation element for evaluating the received search beam and a control element for controlling the transceiver devices. The target objects have the disadvantage that it is not possible to identify and to differentiate between the target objects. The visible or infrared transmission beams have the same wavelength for several target objects in a surveying system.
A method for identifying a target object with the aid of a laser measuring device as well as a corresponding surveying system including a laser measuring device and multiple identifiable target objects are known from DE 10 2010 024 014 A1. The laser measuring device includes a laser measuring unit which transmits a measuring beam and which is designed for measuring a distance and/or an angle, for example, a camera unit as a detection device for detecting the target objects and a control device including a control element for controlling the laser measuring unit and the camera unit. The target object includes a reflector including at least one reflector element for reflecting the measuring beam, a transmitter including at least one transmitter element for transmitting a visible transmission beam, which is used to identify the target object, and a control device including a control element for controlling the transmitter. The identification of the target objects takes place with the aid of the visible transmission beam which is used to identify the target objects and which is adjustable between a first and a second optical state. In the case of the first and the second optical states of the visible transmission beam, different wavelengths or polarization states may be involved, for example. As a further possibility to implement two different optical states, it is provided to switch on (first optical state) and to switch off (second optical state) the visible transmission beam. The laser measuring device is connected or at least designed to be connectable to the target objects of the surveying system via communication links. To establish the communication links, corresponding transceiver elements are provided in the laser measuring device and in the target objects.
The known method for identifying the target objects includes the following method steps: The control device of the target object transmits a control instruction to the transmitter which transmits a visible transmission beam in a first optical state as a result of the control instruction. The visible transmission beam is detected by the receiver of the measuring device and the received part of the transmission beam is evaluated by the control device of the laser measuring device. The control device of the laser measuring device transmits a control instruction to the transmitter of the target object via the communication link, the transmitter switching the visible transmission beam over into the second optical state as a result of the control instruction. The visible transmission beam in the second optical state is detected by the receiver of the laser measuring device and evaluated by the control device of the laser measuring device. The camera unit records a sequence of two-dimensional images of the target object as a result of the control instruction. The images are evaluated by the control device with the aid of known image processing techniques. The control device forms a differential image between the camera image having the transmission beam in the first optical state and the camera image having the transmission beam in the second optical state; the target object may be recognized and identified in the differential image. It is disadvantageous that the laser measuring device and the target object must be connected via a communication link in order to identify the target object. Moreover, the target object must be oriented at least roughly in the direction of the laser measuring device, so that the camera unit of the laser measuring device detects the target object.